Fabrication of an integrated circuit involves processes that can generally be categorized as deposition, patterning, and doping. With the use of these different processes complex structures having various components may be built to form the complex circuitry of a semiconductor device.
Lithography is the formation of a three-dimensional patterning on the substrate to form a pattern to the substrate. A multiplicity of lithographic procedures combined with etching and/or polishing may be performed to create a final semiconductor device.
Photolithography or optical lithography involves the use of a light sensitive polymer or a photoresist that is exposed and developed to form three-dimensional patterning on a substrate. The parts of the substrate that remain covered with the photoresist will be protected from subsequent etching, ion implantation, or certain other processing techniques.
The general sequence for a photolithography process may include the steps of preparing the substrate, applying a photoresist, prebaking, exposing, post-exposure baking, developing, and post-baking. Photoresists may be applied to the substrate by any number of techniques. Generally, it is somewhat important to establish a uniform thickness of the photoresist across the substrate. Optionally, a layer of bottom anti reflectivity coating (BARC) may be applied to the substrate prior to the application of the photoresist layer. Adhesion promoters may be typically applied to the substrate prior to application of the photoresist.
In one exemplary process, the photoresist is applied to the substrate, and the substrate is then spun on a turntable at a high speed to produce the desired film. This process is known as spin coating. Without intending to be bound by theory, the thickness of the photoresist is inversely proportional to the square root of the spin speed and is directly proportional to the viscosity of the photoresist. Thus greater spin speeds lead to a thinner photoresist layer while a more viscous photoresist material leads to a thicker photoresist layer.
A photoresist that has been applied to the substrate will contain a solvent that will eventually evaporate from the photoresist. Of course, the rate of solvent removal may be accelerated by baking the photoresist. Once solvent has been partially removed from the photoresist, the adhesion of the photoresist to the substrate is improved and the photoresist layer becomes more durable. Some residual amount of solvent remaining in the photoresist—on the order of between about 3 and about 8 percent by weight—will allow the photoresist layer to remain stable during subsequent lithographic processing steps.
The premise behind photolithography is the change in solubility of the positive photoresist in a positive tone developer throughout certain regions of the photoresist that have been exposed to light, in the past visible light but more conventionally ultraviolet light, or some other form of radiation. The regions of exposure may be controlled, for example, with the use of a mask. FIG. 1A shows an exposure step of a photolithographic process. Light 10 passing through a mask 20 may only pass to certain regions, the exposed regions 30, of a positive photoresist while leaving other unexposed regions 40 in the positive tone developer (PTD) process and leaving exposed regions 30 in the negative tone developer process (NTD) process.
Exposed regions of a positive photoresist become more soluble in the PTD, while exposed regions of the positive photoresist become less soluble in a negative tone developer. By way of a non-limiting example, the positive photoresist may comprise at least one photoactive compound that is itself converted or causes the conversion of another compound to a compound that is more soluble in a PTD. By way of another non-limiting example, the positive photoresist may comprise a photoactive compound that causes an unprotected polymer to be available after exposure. The unprotected polymer will become hydrophilic and more polar, which makes it less soluble in an organic non-polar solvent.
The exposed regions 30 of a positive photoresist will be washed away by a positive tone developer leaving the unexposed regions 40 of the photoresist that have not been exposed to light. FIG. 1B illustrates a positive tone developer (PTD) patter produced from the use of a positive photoresist.
In contrast, the exposed regions 30 of the positive photoresist will remain after being contacted with a negative tone developer while the unexposed regions 40 of the positive photoresist will be washed away by the NTD. FIG. 1C illustrates a negative tone developer (NTD) image produced from the use of a positive photoresist.
Of course, the developer may be selected based upon the extent of solubility of the positive photoresist. In non-limiting examples, an aqueous 2.38 wt % tetra-methyl ammonium hydroxide solution may be used as the positive tone developer, while an organic solvent may be used as the negative tone developer.
Prior to undergoing development, the photoresist may be subjected to a post-exposure bake that may assist with smoothing standing wave ridges that have formed in the photoresist. Commonly, most positive photoresists employ the use of aqueous bases as positive tone developers. A post-bake follows development and is used to harden the final resist image allowing the remaining photoresist to better withstand subsequent etching or other processing steps. Following the subsequent processing steps, the remaining photoresist is removed from the substrate using, for example, a wet stripping process or a dry plasma stripping process.
Double patterning involves performing two lithographic processing steps. Without intending to be limiting, double patterning may be used in fabricating fine but dense patterns in a semiconductor devices. Double patterning may be particularly useful when optical resolution restrictions prevent achieving a reduced pitch in a semiconductor device. However, conventional approaches to double patterning may have issues with respect to their ability to process a semiconductor device having varying densities. Furthermore, conventional double patterning techniques may also lead to uneven or inaccurate pattern profiles depending upon the design of the semiconductor device.
There remains a need in the art for improved methods of double patterning to further reduce pitch and yet overcome many of the problems associated with more conventional double patterning processes. Furthermore, there remains a need for improved pattern designs to achieve reduced pitch in semiconductor devices.